Injection molding is a well-known process that involves melting solid thermoplastic molding material within a heated machine barrel assembly mounted on a moveable carriage, and transmitting the molding material into a cavity formed within an injection mold. The molding material takes on the shape of the cavity and then solidifies to form a finished part. The mold then opens and the process may be repeated to form additional parts.
The molding material may be solid, semi-rigid, or molten at various stages of an injection molding operation, and while a nozzle tip (the heated machine barrel assembly includes a nozzle having a tip that engages a complimentary nozzle seat formed in the injection mold) may contain molding material in any or all of these physical states, there will always be at least some semi-rigid material located between solid and molten molding material. Crystalline thermoplastic materials will typically have a small amount of semi-rigid material between the other two states, while amorphous thermoplastic materials typically contain a considerably larger amount of semi-rigid material. Control of these three physical states and their location within the nozzle tip is critical to the performance of the molding process.
Shear induced melt imbalance is a common challenge in injection molding runner systems. In both cold and hot runner molds, molding material travels through a bore or hole as it moves between the molding machine nozzle tip and the mold cavity. During this process, molding material contacts the sides of the bore or hole, which increases in temperature as a result of shear, while the temperature in the center of the melt stream remains fairly constant. When the molten molding material makes a 90° turn or comes to a “T,” the hotter, less viscous molding material may be moved to one side. This can result in the mold cavities filling at different rates, as well as one side of a given mold cavity filling at a different rate than the other side. A need exists for a device that can overcome this difference and avoid the resultant nonuniformity among molded parts or within sections of the same part.